You could try Julian, CA. (4,000' elev.). I was there in April 2017, days were in the 70's°, nights in the 40's°. Pinezantita RV Park was very nice with lots of shade trees. Julian is about 60 miles east of San Diego. If the desert is more to your liking, try Borrego Springs, not nearly as busy as Palm Springs.
Of course, you can hit Joshua Tree NP, Mojave NP, Death Valley NP.
If you do the eastern Sierras, also try Convict Lake. There is a RV Park right there with a very good restaurant $$$ on site. Even in April, the nights can be in the 30's°.
If you go thru Bishop, CA. be sure to stop at Erick Schat's Bakery.
Have fun.